1. Field of the Invention
The present invention is directed generally to quantum well infrared photodetector focal plane arrays (QWIP FPA""s) and, more particularly, to QWIP FPAs that are capable of multicolor detection.
2. Background Information
Multicolor infrared detection capability has wide applicability to a number of different applications including aerospace, medical, surveying, mining, and agriculture. For example, multicolor infrared detection can provide useful information during geographical surveys. Detection in at least two infrared (IR) spectral bands would permit differentiation between man-made structures and natural landscape in a geographical survey. Additionally, in medical applications, multi-color detection would permit improved thermal imaging of the human body for diagnostic purposes.
Multicolor infrared detection has been conventionally performed using a wide-band IR detector and an associated rotating mechanical multicolor filter wheel. The wide-band IR detector detects a broad range of incident wavelengths. The rotating filter wheel selects the desired wavelength that is to be passed to the wide-band detector. An exemplary mechanical color filter wheel system is disclosed in U.S. Pat. No. 5,300,780. Mechanical color wheel systems, however, suffer from a number of deficiencies in multicolor detection. Such systems generally are slow and bulky, require large amounts of power for operation, and have a limited life span. Additionally, color wheel systems tend to have poor photon collection efficiency.
To alleviate some of the known deficiencies of mechanical color filter wheel systems, quantum well photodetectors have been constructed that permit the detection of more than one spectral band. Such detectors are described, for example, in U.S. Pat. Nos. 5,013,918, 5,198,659, and 5,384,469. These patents generally disclose the use of a plurality of quantum well sets within a single detector where the frequency response characteristics of each quantum well in the set are tailored by adjusting the well depths. The detectors in these patents are designed for use with a single bias voltage that is applied across all the quantum wells of the set. Thus, each quantum well that is xe2x80x9ctunedxe2x80x9d to a specific frequency band can not have its bias adjusted independently of any other xe2x80x9ctunedxe2x80x9d quantum well in the set. The output voltage responses for each of the xe2x80x9ccolorsxe2x80x9d in the detector will therefore be nonuniform for any given background photon flux. In addition, the detectors of these patents provide only a single detected output signal composed of multi-spectral frequency components. Extraction of different frequency bands from the single output therefore requires additional filtering.
Accordingly, there exists a need in the art for a quantum well photodetector that has multi-color capability but which can also provide independent bias adjustment for each color and which can independently and simultaneously process the outputs of each of the colors of the detector.
Exemplary embodiments of the present invention use a vertically stacked quantum well infrared detector where each xe2x80x9ctunedxe2x80x9d quantum well of the detector can be biased separately from any other quantum well of the detector. The vertically stacked detector can include three or more quantum well layers that are xe2x80x9ctunedxe2x80x9d to different peak wavelengths to permit detection of infrared radiation of three or more different bands or colors. The simultaneous detection of infrared radiation in three or more different spectral bands permits the determination of more information about an infrared source.
The present invention is directed to a photodetector comprising: a first photosensitive layer; a second photosensitive layer; a third photosensitive layer, wherein said photosensitive layers are formed adjacent one another; and means for independently biasing said first, second, and third photosensitive layers. The present invention is additionally directed to a method of fabricating such a photodetector and fabricating a focal plane array including a plurality of such photodetectors.
In alternate embodiments, a photodetector according to the present invention comprises: first means responsive to impinging energy of a first spectral band for providing a first quantity of moving charges to a first output of said photodetector, second means responsive to impinging energy of a second spectral band for providing a second quantity of moving charges to a second output of said photodetector, wherein said impinging energy of said second spectral band first passes through said first means before reaching said second means; and third means responsive to impinging energy of a third spectral band for providing a third quantity of moving charges to a third output of said photodetector, wherein said impinging energy of said third spectral band first passes through said second means before reaching said third means.